Process for plating a carbon or graphite part with chromium



Patented Apr. 3, 1951 PROCESS FOR PLATING A CARBON OR GRAPHITE PART WITH CHROMIUM Robert Seymour Herwig, Arlington, N. J assignor to Bendix Aviation Corporation, Teterboro, V N. J a corporation of. Delaware No Drawing. Application October 23, 1945,

Serial No. 624,090

3 Claims. (Cl. 204 51) "The present application relates to an electro- Iytic coating process for a carbon product and more particularly to a process for plating a car- .bon or graphite part with chromium. Heretof ore, because of their unique properties,

a great number of uses have been applied to carbon and graphite products, particularly as ,electrical'brushes, bearings, liners, bushings, re-

sistors, washers and seals. There are, however,

-certain disadvantages to such use of carbon .products, namely, their comparative brittleness ;:and low surface hardness. ;advantage;s;have been somewhat overcome by im- In the past such dispregnating the carbon or graphite with metals or by combining the carbonor graphite powder with metal powders, compressing and then heating to the desired temperature for preparation of the part.

An object of the present invention is to provide a novel process for increasing the surface hardness of a carbon or graphite part by plating the surface thereof with a metal such as chromium.

The attempts of the applicant to electrolyticalv.ly plate chromium on carbon or graphite by the standard hard chromium plating process heretofore employed with other metals resulted in no plating, or at the best, a plate on the outer edges only. It was found, moreover, that the increasing of the voltage applied in the latter h process .after initial energization merely increased the amount of gassing without any fur- Other objects and advantages of this invention areset forth in the following description and the novel features thereof are in the appended claims.

pointed o'ut The disclosure, however, is illustrative only and changes may be made therein within the principle of the invention to the full extent indicated by the meaning of the terms in which the appended claims are expressed.

Specifically, as an example of the invention, a carbon or, graphite brush or other carbon prod- 1 not is first subject to a cleaning process by soaking the carbon product in a suitable alkaline cleaner. The cleaner bath may, for example,

consist of a water solution containing three ounces vof sodium hydroxide (NaOH) and two ounces of sodium carbonate (NazCOs) per gallon of. water.

A te see in th ca bo ro uct .in. he bath for a suitable period, the cleaning step should be followed by a short reverse current clean for a few seconds of between 5 to 15 seconds during which time the carbon product is positively charged, while another pole immersed in the bath is negatively charged in relation thereto so as to assure a further cleaning action. The temperature of the bath should be maintained at one hundred and sixty degrees Fahrenheit F.) and the reverse current clean should be limited to a few seconds as higher temperatures or a reverse current clean fora long period will cause excess surface oxidation resulting in a poor bond between .the chromium and the carbon product. U

The carbon product after subjection to the aforenoted cleaning process is removed and then immersed in an electrolyte consisting of 33 ounces of chromic acid and. .33 ounce of concentrated sulfuric acid per gallon of water.

In the electrolyte the carbon product forms a cathode, while another pole immersed in the electrolyte is positively charged in relation to the carbon product.

In this connection, it is necessary that the greatest precaution be used in wiring the carbon product for chromium plating as the high resistance of carbon and the small contact area between the connecting wire and the carbon causes an appreciable voltage drop between the electrical connecting wire and the carbon product. Whenever possible the carbon product should be securely bolted to the connecting wire or otherwise securely fastened thereto.

Another important factor in the chromium -plating of carbon and graphite is that the connecting wire holding the carbon product in the electrolyte must be stopped off or insulated from the plating bath by a suitable coating or covering material.

This is necessitated by the fact that the wires connecting the carbon to the source of electrical energy will otherwise act as a thief, and divert the flow of chromium or metal ions in the electrolyte from the carbon, due to the relatively higher resistance of the carbon or graphite to the flow of electrical energy and the tendency of the metal ions which carry the current through the electrolyte to travel through the path of least resistance.

In order to effectively plate chromium on a fcarbon product a minimum of fifty amperes is fifteen degrees (115 F.) to one hundred and thirty degrees Fahrenheit (130 F.).

In the development of the present plating process one of the factors that caused considerable difficulty in the plating of carbon products with chromium was the calculation of the efiec'-" tive area. Thus, for example, in the case of a block of carbon or graphite one inch square, the apparent effective area would be six square inches. Actually this is not the case, since there can be anywhere from six square inches to sixty square inches or more effective area, dependent on the carbon particles used and the porosity of the resulting block. This fact reveals why in the present improved plating process relatively high voltages are required to assure the necessary minimum current densities for effectively plating the carbon product.

If the total of the electrical resistance of the electrolyte, carbon, and electrical connecting wires is such that the voltage first used is sufficient to provide the minimum fifty amperes per square foot of effective surface area of the carbon and preferably a greater amperage should ,beused, then the plating takes place.

However, if the first surge of plating current does not give at least the minimum amperage required, then the hydrogen gas Which immediately forms at the cathode prevents further plat- In order to reduce the electrical resistance afforded by the electrolyte to the passage of the plating current, the anode and the cathode (the carbon product) are placed a sufficient distance apart so as to prevent shorting, while sufficiently close so as to provide a resistance not in excess of that which would effect at least the minimum amperage aforenoted and an initial plating of the carbon at the voltage applied.

If the anode and cathode (or carbon product) be placed too far apart, an excess hydrogen will form at the cathode, polarising the carbon and preventing the plating action by forming a film of hydrogen bubbles.

In order to obtain optimum plating conditions I have found it is necessary to move the anode to within of an inch to of an inch of the carbon part. This introduces difliculties, however, in that the placing of the anode so close to the part being plated may cause shorts and burning of the carbon. In order to avoid the latter condition, the carbon part should be placed on self-contained anode racks so as to minimize any danger of a short due to the carbon part touching the anode during the plating process.

I have found, moreover, that for best results the initialvoltage applied to the anode and catho'de' should be relatively high so as to assure the plating action at an amperage considerably above the minimum aforenoted, but of course not so high as to cause cracking of the plating.

In my experiments, I have found that in the case of a carbon part having an apparent surface area of about four square inches, that is, in size 1 inch x 1 inch X inch that an applied voltage of eight volts during the initial energization of the plating circuit causes good plating conditions under the particular circuit resistance employed in which the resistance is kept to a minimum as previously noted. Under such conditions, it will. be seenv that the initial plating current would considerably exceed the minimum 50 amperes condition previously noted.

This ,-high initial plating voltage should be maintained for only a: few minutes or approxi- 4 mately two minutes until complete coverage of the carbon part with chromium is obtained. The voltage is then decreased to four or five volts for thirty minutes and further decreased to three volts for one hour and a half.

This combination of voltages resulted in a bright chromium deposit of approximately .002" thick, measured with a metallograph. The res'ulting deposit may be cut and ground without chipping of the bond between the chromium and the carbon.

It is most important, however, that the higher voltage (8 volts) should not be used for too long a period of time, as the chromium deposit that occurs at the higher current densities is more highly stressed due to the occluded hydrogen gas in the chromium plate and chromic oxides which cause lifting and severe cracking of the chromium plate.

Summing up the foregoing, it would appear that the following rules should be observed'for plating carbon and graphite directly with chromium:

1. Clean the carbon or graphite product carefully by soaking in a mild alkaline cleaner followed by a short reverse clean.

2. Wire as securely as possible.

.3. Stop offall exposed parts that arenst to be plated. 5

4. Place the cathode carbon part in a chromium plating electrolyte and within a short distance of the anode or at a distance of at least of an inch and not more than of an inch from the anode.

5. Initially pass a relatively high plating current through the circuit until complete plating coverage is acquired.

6. Reduce plating current to a lower amperage of at least the minimum aforenoted and plate to desired thickness. v

7. The plating amperage should not fall below the minimum of 50 amperes per square foot of efiective area of the carbon part.

The use of the relatively higher voltage in the initial energization of the plating circuit is due in part to the high resistance afforded by the carbon part and the great effective surface area of the carbon part caused by the carbon particles which make up the same as previously described. After initial chromium plating is effected, the latter plating coat in effect reduces the effective surface area of the carbon part and may partially shunt the carbon from the plating circuit. It is of the utmost importance to provide a voltage sufiiciently high so as to effect the initial plating action with the first surge of plating current or otherwise plating will not take place as the excess hydrogen which would result from too low an initial plating voltage would preventja chromium plate from being later effected.

After the initial chromium plating is effected on the carbon part, continued chromium plating results in effect by plating chromium on the initial chromium plate.

voltages in each case should be determined for the particular carbon part involved.

In any given case, however, the initial plating voltage or current should be sufficiently great as to effect an immediate plating action with the first surge of plating current, but should not be so great as to cause a lifting or cracking of the carbon or plate and, moreover, after the initial plating has been effected this voltage or current should be reduced for the modified effective plating surface area so that the plating action may continue without cracking the chromium plate initially formed on the carbon part.

It is to be understood, however, that the conditions of temperature, voltage, concentrations and ingredients disclosed herein are not limita- 'tions of the invention, but are to be considered merely as illustrative of the practice thereof, since the invention is not to be limited except by the appended claims,

What is claimed is:

1. A process for chromium plating a carbon part; comprising the steps of first soaking the carbon part in an alkaline cleaning solution including three ounces of sodium hydroxide and two ounces of sodium carbonate per gallon of water; second subjecting said carbon part to a reverse current clean for a period, of between five to fifteen seconds and at a temperature of one hundred and sixty degrees Fahrenheit; third passing an electric current through the carbon part at a current density considerably in excess of a rate of fifty amperes for each square foot of effective surface area of the carbon part and at a relatively high voltage while the carbon part is functioning as a cathode in a Cr O3-SO4 acid plating bath and in which the carbon part is separated from the anode at a distance of from one sixteenth of an inch to a half of an inch; fourth reducing the voltage initially applied to the carbon part after initial chromium plating has been effected and plating at a current density less than before, but at an amperage rate of at least fifty amperes for each square foot of effective surface area of the part.

2. A process for chromium plating a carbon part; comprising the steps of passing an electric current through the carbon part at a current density considerably in excess of a rate of fifty amperes for each square foot of effective surface area of the carbon part and at a relatively high 6 voltage while the carbon part is functioning as a cathode in a Cr 03-SO4 acid plating bath and in which the carbon part is separated from the anode at a distance of from one sixteenth of an inch to a half of an inch; and reducing the voltage initially applied to the carbon part after initial chromium plating has been effected and plating at a current density less than before, but at an amperage ratejof at least fifty amperes for each square foot of effective surface area of the part. 7

3. A process for chromium plating a carbon part having an apparent surface area of four square inches; comprising the steps of placing the carbon part in a chromium plating acid bath and within one-sixteenth of an inch to one-half of an inch of an anodaapplying a negative charge to said carbon part relative to said anode at a voltage of eight vol-ts, so as to effect a plating current at a rate in '"excess of fifty amperes per square foot of effective area of the carbon part for approximately two minutes so as to effect complete coverage of the carbon part with chromium; then decreasing the voltage to between four or five volts'for thirty minutes and then further decreasing the voltage to three volts for an hour and a half while maintaining the plating current at arate not less than the minimum of fifty amperes per square foot of effective area of the carbon part.

ROBERT SEYMOUR HE'RWIG.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED sTATEs PATENTS OTHER REFERENCES Transactions of the Electrochemical Society, vol. (1941), pp. 589 and 593. 

1. A PROCESS FOR CHROMIUM PLATING A CARBON PART; COMPRISING THE STEPS OF FIRST SOAKING THE CARBON PART IN AN ALKALINE CLEANING SOLUTION INCLUDING THREE OUNCES OF SODIUM HYDROXIDE AND TWO ONCES OF SODIUM CARBONATE PER GALLON OF WATER; SECOND SUBJECTING SAID CARBON PART TO A REVERSE CURRENT CLEAN FOR A PERIOD OF BETWEEN FIVE TO FIFTEEN SECONDS AND AT A TEMPERATURE OF ONE HUNDRED AND SIXTY DEGREES FAHRENHEIT; THIRD PASSING AN ELECTRIC CURRENT THROUGH THE CARBON PART AT A CURRENT DENSITY CONSIDERABLY IN EXCESS OF A RATE OF FIFTY AMPERS FOR EACH SQUARE FOOT OF EFFECTIVE SURFACE AREA OF THE CARBON PART AND AT A RELATIVELY HIGH VOLTAGE WHILE THE CARBON PART IS FUNCTIONING AS A CATHODE IN A CR O3-SO4 ACID PLATING BATH AND IN WHICH THE CARBON PART IS SEPARATED FROM THE ANODE AT A DISTANCE OF FROM ONE SIXTEENTH OF AN INCH TO A HALF OF AN INCH; FOURTH REDUCING THE VOLTAGE INITIALLY APPLIED TO THE CARBON PART AFTER INITIAL CHROMIUM PLATING HAS BEEN EFFECTED AND PLATING AT A CURRENT DENSITY LESS THAN BEFORE, BUT AT AN AMPERAGE RATE OF AT LEAST FIFTY AMPERES FROM EACH SQUARE FOOT OF EFFECTIVE SURFACE AREA OF THE PART. 